Building with earth - Gernot MINKE (1)

ing force according to the standard DIN
18952 test (see p. 32), with a slight change:
here the maximum strength of five samples
was also considered.
This is because it was found that the lower
values were usually due to insufficient mixing,
inaccurate plasticity or other preparation
mistakes. In order to guarantee that different
loam mixtures are comparable, the chosen
consistency of the samples was defined
by a diameter of 70 mm (instead of 50 mm)
of the flat circular area, which forms if a test
ball of 200 g weight is dropped from a
height of 2 m. (With sandy loam mixtures
with little clay content, a diameter of 50 mm
is not attainable.)
Acid test
Loams that contain lime are normally white
in appearance, exhibit a low binding force
and are therefore inappropriate for earth
construction. In order to define the lime
content, one drop of a 20% solution of HCl
is added using a glass or a timber rod. In
the case of loam with lime content, CO 2 is
produced according to the equation CaCO 3
+ 2HCl = CaCl 2 + CO 2 + H 2 O. This CO 2 production
is observable because of the efflorescence
that results; if there is no efflorescence,
the lime content is less than 1%. If
there is a weak, brief efflorescence, the lime
content is between 1% and 2%; if the efflorescence
is significant though brief, the lime
content is between 3% and 4%; and if the
efflorescence is strong and long lasting, the
lime content is more than 5% (Voth, 1978,
p. 59).
It should be noted that a dark lime-free
loam with a high content of humus could
also exhibit this phenomenon.
disadvantageous for its use as a building
material. Swelling only occurs if loam comes
into direct contact with so much water that
it loses its solid state. The absorption of
humidity from the air, however, does not
lead to swelling.
The amount of swelling and shrinkage
depends on the type and quantity of clay
(with Montmorillonite clay this effect is
much larger than with Kaolinite and Illite),
and also on the grain distribution of silt and
sand. Experiments were conducted at the
BRL using 10 x 10 x 7 cm samples of different
loam mixtures that were soaked with
80 cm 3 of water and then dried in an oven
at 50°C in order to study shrinkage cracks
(2.13). Industrially fabricated unbaked blocks
(2.13, top left), whose granularity curve is
shown in 2.1 (upper left), display shrinkage
cracks. A similar mixture with the same kind
and amount of clay, but with ”optimised“
distribution of silt and sand, exhibited hardly
any cracks after drying out (2.13, top right).
The mud brick made of silty soil (2.13, bottom
right) (granularity curve shown in 2.1,
middle) shows several very fine cracks,
whereas the mud brick of sandy soil (2.13,
bottom left) (granularity curve shown in 2.1,
bottom) shows no cracks at all. On p. 39
it is explained how shrinkage might be minimised
by changing grain distribution.
Determining linear shrinkage
Before the shrinkage ratio of different loam
samples can be compared, they must have
comparable plasticity.
The German standard DIN 18952 describes
the following steps required to obtain this
standard stiffness:
2.10
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0 20 40 60 80 100
Ribbon rupture lenght (cm)
2.12
2.10 Ribbon test
2.11 Cohesion test developed
at the BRL
2.12 Binding force of
different loams of equal
consistency in relation
to their rupture lengths,
tested according to the
BRL cohesion test
Effects of water
If loam becomes wet, it swells and changes
from a solid to a plastic state.
Swelling and shrinking
The swelling of loam when in contact with
water and its shrinkage through drying is
24
Properties of earth
2.11